We have recently isolated a partial cDNA encoding a protein which binds to the carboxy-terminal tail of a membrane receptor known as gp330/megalin. gp330 is a large glycoprotein receptor that has been shown to be the antigen involved in Heymann nephritis, a rat model of human membranous glomerulonephritis. The expression of gp330 is only expressed on the apical surface of epithelial cells such as the kidney proximal tubule brush border. The truncated protein which we have isolated contains a recently identified protein interaction domain known as the PDZ domain. This domain has been shown to bind to an amino acid sequence motif found at the extreme carboxy-terminus of cell surface receptors. gp330 contains such a motif and it is believed that the truncated protein, which we have named GASP, may be a physiological binding partner to gp330. Most proteins containing PDZ domains function in localizing receptor and channel proteins to distinct areas of a cell such as neuronal synapses and cell-to-cell contacts. We will be cloning the full length GASP cDNA in an attempt to learn more about the proteins function through the identification of other protein interacting domains. In addition, we will be characterizing the interaction of GASP with gp330 at the molecular level in an attempt to provide evidence that GASP is involved in the localization and function of gp330 within the kidney epithelia.